Laminated dielectric filter

Abstract: A laminated dielectric filter comprises plural strip line resonators and a capacity electrode layer.

Description: This invention relates to a laminated dielectric filter used mainly for dielectric antenna duplexers in high frequency radio devices such as mobile telephones. An antenna duplexer is a device for sharing one antenna by a transmitter and a receiver, and it is composed of a transmission filter and a reception filter. The invention is particularly directed to a laminated dielectric filter having a laminate structure by laminating a dielectric sheet and an electrode layer and baking into one body. Along with the advancement of mobile communications, recently, the antenna duplexer is used widely in many hand-held telephones and car-mounted telephones. An example of a conventional antenna duplexer is described below with reference to a drawing. Fig. 27 is a perspective exploded view of a conventional antenna duplexer. In Fig. 27, reference numerals 701 to 706 are dielectric coaxial resonators, 707 is a coupling substrate, 708 is a metallic case, 709 is a metallic cover, 710 to 712 are series capacitors, 713 and 714 are inductors, 715 to 718 are coupling capacitors, 721 to 726 are coupling pins, 731 is a transmission terminal, 732 is an antenna terminal, 733 is a reception terminal, and 741 to 747 are electrode patterns formed on the coupling substrate 707. The dielectric coaxial resonators 701, 702, 703, series capacitors 710, 711, 712, and inductors 713, 714 are combined to form a transmission band elimination filter. The dielectric coaxial resonators 704, 705, 706, and coupling capacitors 715, 716, 717, 718 compose a reception band pass filter. One end of the transmission filter is connected to a transmission terminal which is electrically connected with a transmitter, and the other end of the transmission filter is connected to one end of a reception filter, and is also connected to an antenna terminal electrically connected to the antenna. The other end of the reception filter is connected to a reception terminal which is electrically connected to a receiver. The operation of an antenna duplexer is described below. First of all, the transmission band elimination filter shows a small insertion loss to the transmission signal in the transmission frequency band, and can transmit the transmission signal from the transmission terminal to the antenna terminal while hardly attenuating it. By contrast, it shows a larger insertion loss to the reception signal in the reception frequency band, and reflects almost all input signal in the reception frequency band, and therefore the reception signal entering from the antenna terminal returns to the reception band pass filter. On the other hand, the reception band filter shows a small insertion loss to the reception signal in the reception frequency band, and transmits the reception signal from the antenna terminal to the reception terminal while hardly attenuating it. The transmission signal in the transmission frequency band shows a large insertion loss, and reflects almost all input signal in the transmission frequency band, so that the transmission signals coming from the transmission filter is sent out to the antenna terminal. In this design, however, in manufacturing dielectric coaxial resonators, there is a limitation in fine processing of ceramics, and hence it is hard to reduce its size. Downsizing is also difficult because many parts are used such as capacitors and inductors, and another problem is the difficulty in lowering the assembling cost. The dielectric filter is a constituent element of the antenna duplexer, and is also used widely as an independent filter in mobile telephones and radio devices, and there is a demand that they be smaller in size and higher in performance. Referring now to a different drawing, an example of a conventional block type dielectric filter possessing a different constitution from the above described structure is described below. Fig. 28 is a perspective oblique view of a block type dielectric filter of the prior art. In Fig. 28, reference numeral 1200 is a dielectric block, 1201 to 1204 are penetration holes, and 1211 to 1214, and 1221, 1222, 1230 are electrodes. The dielectric block 1200 is entirely covered with electrodes, including the surface of the penetration holes 1201 to 1204, except for peripheral parts of the electrodes on the surface of which the electrodes 1221, 1222 and others are formed. The operation of the thus constituted dielectric filter is described below. The surface electrodes in the penetration holes 1201 to 1204 serve as the resonator, and the electrode 1230 serves as the shield electrode. The electrodes 1211 to 1214 are to lower the resonance frequency of the resonator composed of the electrodes in the penetration holes, and functions as the loading capacity electrode. By nature, a 1/4 wavelength front end short-circuit transmission line is not coupled at the resonance frequency and shows a band stop characteristic, but by thus lowering the resonance frequency, an electromagnetic field coupling between transmission lines occurs in the filter passing band, so that a band pass filter is created. The electrodes 1221, 1222 are input and output coupling capacity electrodes, and input and output coupling is effected by the capacity between these electrodes and the resonator, and the loading capacity electrode. The operating principle of this filter is a modified version of a comb-line filter disclosed in the literature (for example, G.L. Matthaei, "Comb-Line Band-pass Filters of Narrow or Moderate Bandwidth"; the Microwave Journal, August 1963). The block type filter in this design is a comb-line filter composed of a dielectric ceramic (for example, see U. S. Patent 4,431,977). The comb-line filter always requires a loading capacity for lowering the resonance frequency in order to realize the band pass characteristic. Fig. 29 shows the transmission characteristic of the comb-line type dielectric filter in the prior art. The transmission characteristic shows the Chebyshev characteristic increasing steadily as the attenuation outside the bandwidth departs from the center frequency. In this construction, however, it is not possible to realize the elliptical function characteristic possessing the attenuation pole near the bandwidth of the transmission characteristic, and hence the range of selection is not sufficient for filter performance. Also, in such dielectric filter, for smaller and thinner constitution, the flat type laminate dielectric filter that can be made thinner than the coaxial type is expected henceforth, and several attempts have been made to design such a device. A conventional example of a laminated dielectric filter is described below. The following explanation relates to a laminated "LC filter" (trade mark) that is put into practical use as a laminated dielectric filter by forming lumped element type capacitors and inductors in a laminate structure. Fig. 30 is a perspective exploded view showing the structure of a conventional laminate "LC filter". In Fig. 30, reference numerals 1 and 2 are thick dielectric layers. On a dielectric sheet 3 are formed inductor electrodes 3a, 3b, and capacitor electrodes 4a, 4b are formed on a dielectric sheet 4, capacitor electrodes 5a, 5b on a dielectric sheet 5, and shield electrodes 7a, 7b on a dielectric sheet 7. By stacking up all these dielectric layers and dielectric sheets together with a dielectric sheet 6 for protecting the electrodes, an entirely laminated structure is formed. The operation of the thus constituted dielectric filter is described below. First, the confronting capacitor electrodes 4a and 5a, and 4b and 5b respectively compose parallel plate capacitors. Each parallel plate capacitor functions as a resonance circuit as connected in series to the inductor electrodes 3a, 3b through side electrodes 8a, 8b. Two inductors are coupled magnetically. The side electrode 8b is a grounding electrode, and the side electrode 8c is connected to terminals 3c, 3d connected to the inductor electrode to compose a band pass filter as input and output terminals (for example, JP-A-3-72706(1991)). In such a constitution, however, when the inductor electrodes are brought closer to each other to narrow the interval in order to reduce in its size, the magnetic field coupling between the resonators becomes too large, and it is hard to realize a favorable band pass characteristic narrow in the bandwidth. It is moreover difficult to heighten the unloaded Q value of the inductor electrodes, and hence the filter insertion loss is large. Another different conventional example of a laminated dielectric filter is described below with reference to an accompanying drawing. Fig. 31(a) and (b) shows the structure of a conventional laminated dielectric filter. In Fig. 31(a) and (b), 1/4 wavelength strip lines 820, 821 are formed on a dielectric substrate 819. Input and output electrodes 823, 824 are formed on the same plane as the strip lines 820, 821. The strip line 820 is composed of a first portion 820a (L 1 indicates the length of 820a) having a first line width W 1 (Z 1 indicates the characteristic impedance of W 1 ) confronting the input and output electrodes 823, a second portion 820b (L 2 indicates the length of 820b) having a second line width W 2 narrower than the first line width W 1 , and a third portion 820c having a third line width narrower than the first line width W 1 but broader than the second line width W 2 (Z 2 indicates the characteristic impedance of W 2 ). Similarly, the strip line 821 is composed of a first portion 821a having a first line width W 1 confronting the input and output electrodes 824, a second portion 821b having a second line width W 2 narrower than the first line width W 1 , and a third portion 821c having a third line width narrower than the first line width W 1 but broader than the second line width W 2 . The strip lines 820, 821 are connected with a short-circuit electrode 822, and the resonator 801b is in a pi-shape. A dielectric substrate 819 is covered by grounding electrodes 825, 826 at both surfaces. At one side 819a, side electrodes 827,828 are formed, and the grounding electrodes 825, 826, and short-circuit electrodes 822 are connected. On the other side 819b, side electrodes to be connected with the input and output electrodes 823, 824 respectively are formed. The strip lines 820, 821 are capacitively coupled with the input and output electrodes 823, 824, respectively, thereby constituting a filter as described for example, in U. S. Patent 5.248.949. In such constitution, however, same as the conventional block type dielectric filter, the elliptical function characteristic possessing the attenuation pole near the passing band of the transmission characteristic cannot be realized, and hence the scope of performance of the filter is not wide enough. T. Ishizaki et al.: «A very small dielectric planar filter for portable telephones« 1993, IEEE MTT-S INTERNATIONAL MCROWAVE SYMPOSIUM-DIGEST, VOL. 1, 14-18 June 1993, pages 177-180 relates to a very small dielectric planar filter for portable telephones. A high performance dielectric filter with the size of 4.5mm x 3.2mm x 2.0mm has been developed. It has two planar resonators and is made of high permittivity multilayer ceramic in the Bi-Ca-Nb-o system. An equivalent lumped circuit is derived to explain the behaviours of attenuation pole quantitatively. EP-A-0 506 467 discloses a dielectric filter having coupling electrodes for connecting resonator electrodes. A tri-plate type dielectric filter comprises a dielectric substrate , a plurality of resonator electrodes embedded in the substrate, and coupling electrodes formed within the dielectric substrate, for electrically connecting the resonator electrodes, so as to provide capacitors each between adjacent ones of the resonator electrodes. The resonator electrodes may take the form pf parallel elongate strips each providing a strip line type λ/4 or λ/2 TEM mode resonance circuit. One end of each strip is exposed on an outer surface of the substrate. This end of each strip is trimmed to adjust the resonance frequency of the resonance circuit. It is a primary object of the invention to provide a laminated dielectric filter at low cost which has an excellent band pass characteristic with small insection loss and high bandwidth selectivity. Another object is to provide a laminate dielectric filter having a small and thin flat structure. The objects are achieved by the features of the claims. An aspect of the invention provides a laminated dielectric filter comprising a strip line resonator electrode layer forming plural strip line resonators, and a capacity electrode layer, wherein the strip line resonator electrode layer and capacity electrode layer are enclosed by two shield electrode layers, and the space between the two shield electrode layers are filled with a dielectric, and the thickness between the strip line resonator electrode layer and capacity electrode layer is set thinner than the thickness between the strip line resonator electrode layer and shield electrode layer and the thickness between the capacity electrode layer and shield electrode layer. In the laminated dielectric filter of the invention as set forth in this second aspect, by forming a thick dielectric sheet by laminating several thin green sheets, all dielectric sheets can be constituted in the same standardized thickness, and it is easy to manufacture. Moreover, when the dielectric sheet between the shield electrode layer and strip line resonator electrode layer is thick, the unloaded Q value of the resonator is high, and hence a filter of low loss can be realized. It is preferable that the dielectric between the strip line resonator electrode layer and the shield electrode layer, and the dielectric between the capacity electrode layer and the shield electrode layer are respectively formed by laminating a plurality of thin dielectric sheets. It is preferable that the strip line resonator possesses a front end short-circuit structure, and the short-circuit end is connected and grounded electrically to the grounding terminal formed at the side of the dielectric through a broad common grounding electrode formed on the same electrode layer as the strip line resonator electrode layer. In the laminated dielectric filter of the invention, grounding is effected securely, and fluctuations in the resonance frequency due to cutting errors when cutting the dielectric sheet can be reduced. It is preferable that the interstage coupling capacity electrode, or input and output coupling capacity electrode, or loading capacity electrode formed on the capacity electrode layer has a dent shape narrowed in the electrode width in the region overlapping the outer edge of the strip line resonator electrode of the strip line resonator electrode layer. In the laminated dielectric filter of the invention, the dent formed in the capacity electrode enables a reduction in the changes of the area of the overlapping region when position deviation occurs between the strip line resonator electrode layer and capacity electrode layer. As a result, in the manufacturing process, fluctuations of filter characteristics due to deviation of position of the strip line resonator electrode layer and the capacity electrode layer can be suppressed effectively. It is preferable that the laminate dielectric filter possesses an input and output coupling capacity electrode on the capacity electrode layer, and the strip line resonator possesses a front end short-circuit structure, moreover, it is preferable that the input and output coupling capacity electrode and strip line resonator are coupled capacitively at an intermediate position between the open end and short-circuit end of the strip line resonator. It is preferable that the input and output terminals electrically connected to the input and output coupling capacity electrode are formed of side electrodes provided in the lateral direction of the strip line resonator. In the laminated dielectric filter of this embodiment of the invention, by a series resonance circuit comprised of the open end line portion of the strip line resonator and the loading capacitor, an attenuation pole is added to the filter transmission characteristic, and an excellent selection characteristic can be realized. Moreover, the distance between two input and output electrodes can be separated, the spatial coupling between input and output can be reduced, and thus the isolation can be increased. It is preferable that the multiple factor of shrinkage in baking the dielectric is set smaller than the multiple factor of shrinkage in baking the electrode material for making the strip line resonator electrode layer and capacity electrode layer. In the laminated dielectric filter of the invention, a terminal electrode having the electrode terminal formed on the side in a state projected by several microns to scores of microns can be favorably and securely connected to the end face of the laminate. It is preferable that the laminated dielectric filter possesses at least two capacity electrode layers which enclose the strip line resonator electrode layer from above and below. Thus a laminated dielectric filter of small size, low loss, and easy to manufacture can be realized. Fig. 1 is a perspective exploded view of a laminated dielectric filter in a first embodiment of the invention. Fig. 2 is an equivalent circuit diagram of the laminated dielectric filter in the first embodiment of the invention. Fig. 3 is a graph showing the relationship between the even mode impedance step ratio and normalized resonator line length in the laminated dielectric filter in the first embodiment of the invention. Fig. 4 is a graph showing the relationship between the even mode impedance step ratio and even/odd mode impedance ratio in the laminated dielectric filter in the first embodiment of the invention. Fig. 5 is a graph showing the relationship between the even mode impedance and even/odd mode impedance ratio to the structural parameters of a parallel coupling strip line of the invention. Fig. 6 (a) and (b) are graphs showing simulation results of design value of transmission characteristic of the laminated dielectric filter in the first embodiment of the invention. Fig. 6 (a) showing the characteristic of a first trial filter with a low-zero, and Fig. 6 (b) showing the characteristic of a second trial filter with a high-zero. Fig. 7 (a) and (b) are graphs showing the measured value and calculated value of transmission characteristic of the laminated dielectric filter in the first embodiment of the invention, Fig. 7 (a) showing the characteristic of a first trial filter with a low-zero, and Fig. 7 (b) showing the characteristic of a second trial filter with a high-zero. Fig. 8 is a perspective view of a modified form of laminated dielectric filter in the first embodiment of the invention. Fig. 9 is a perspective exploded view of a laminated dielectric filter in a second embodiment of the invention. Fig. 10 is a graph showing the relationship between the loading capacity and the normalized resonator line length in the laminated dielectric filter in the second embodiment of the invention. Fig. 11 is a perspective exploded view of a laminated dielectric filter in a third embodiment of the invention. Fig. 12 is an equivalent circuit diagram of the laminated dielectric filter in the fourth embodiment of the invention. Fig. 13 (a) and (b) are graphs showing the relation between the attenuation frequency and even/odd mode impedance ratio of the laminated dielectric filter in the third embodiment of the invention, Fig. 13 (a) showing the case for a low-zero filter and Fig. 13 (b) showing the case for a high-zero filter. Fig. 14 is a graph showing the relationship of the coupling capacity, the even/odd mode impedance ratio, and normalized resonator line length of the laminated dielectric filter in the third embodiment of the invention. Fig. 15 is a graph showing the relationship of the loading capacity, even/odd mode impedance ratio, and normalized resonator line length of the laminated dielectric filter in the third embodiment of the invention. Fig. 16 (a) and (b) are graphs showing the relationship of the attenuation frequency, coupling capacity, and loading capacity of the laminated dielectric filter in the third embodiment of the invention, Fig. 16 (a) showing the case for a low-zero filter and Fig. 16 (b) showing the case for a high-zero filter. Fig. 17 (a) and (b) are graphs showing the simulation results of transmission characteristic of the laminated dielectric filter of the first embodiment and the laminated dielectric filter in the third embodiment of the invention, Fig. 17 (a) showing the characteristic of the low-zero filter and Fig. 17 (b) showing the characteristic of the the high-zero filter. Fig. 18 (a) is perspective exploded view of a laminated dielectric filter in a fourth embodiment of the invention, and Fig. 18 (b) is a sectional view of section A-A' in Fig. 18 (a). Fig. 19 is an equivalent circuit diagram of the laminated dielectric filter in the fourth embodiment of the invention. Fig. 20 is a perspective layout diagram of an electrode pattern of resonator electrode and capacity electrode of the laminated dielectric filter in the fourth embodiment of the invention. Fig. 21 is a perspective exploded view of a laminated dielectric filter in a fifth embodiment of the invention. Fig. 22 is an equivalent circuit diagram of the laminated dielectric filter in the fifth embodiment of the invention. Fig. 23 is a perspective exploded view of a laminated dielectric filter in sixth embodiment of the invention. Fig. 24 is an equivalent circuit diagram of the laminated dielectric filter in the sixth embodiment of the invention. Fig. 25 is a perspective exploded view of a laminated dielectric filter in a seventh embodiment of the invention. Fig. 26 is an equivalent circuit diagram of the laminated dielectric filter in the seventh embodiment of the invention. Fig. 27 is a perspective exploded view of a dielectric antenna duplexer of the prior art. Fig. 28 is a perspective view of a block dielectric filter of the prior art. Fig. 29 is a graph showing transmission characteristic and reflection characteristic of a comb-line dielectric filter of the prior art. Fig. 30 is a perspective exploded view of a laminated LC filter of the prior art. Fig. 31 (a) and (b) is a perspective view of a laminated dielectric filter of the prior art. An antenna duplexer comprises a combination of a transmission filter and a reception filter. In the following illustrative examples, the individual filters which are used in the antenna duplexer, particularly the laminated dielectric filters are described. EXAMPLE 1 A laminated dielectric filter in a first embodiment of the invention is described below with reference to the drawings. Fig. 1 is a perspective view of a dielectric filter in the first embodiment of the invention. In Fig. 1, reference numerals 10a, 10b are thick dielectric sheets. Strip line resonator electrodes 11a, 11b are formed on the dielectric sheet 10a, and capacity electrodes 12a, 12b are formed on the dielectric sheet 10c. The strip line resonator electrodes 11a, 11b have a SIR (stepped impedance resonator) structure in which the overall line length is shorter than a quarter wavelength composed by the cascade connection of the other ends of first transmission lines 17a, 17b with high characteristic impedance grounded at one end, and second transmission lines 18a, 18b with low characteristic impedance opened at one end. The SIR structure is described in M. Makimoto et al., "Compact Bandpass Filters Using Stepped Impedance Resonators," Proceedings of the IEEE, Vol. 67, No. 1, pp. 16-19, January 1979 and is disclosed in U.S. Patent No. 4,506,241 which are incorporated by reference. It is known in the art that the line length of the resonator can be cut shorter than a quarter wavelength. By contrast, the structure of the invention differs greatly from the prior art in that each resonator has the SIR structure, and the first transmission lines are mutually coupled electromagnetically, and the second transmission lines are mutually coupled electromagnetically, with each electromagnetic field coupling amount set independently by varying the line distance of the transmission lines. The short-circuit end side of the first transmission line is grounded through a common grounding electrode 16. By grounding through the common grounding electrode 16, grounding is done securely, and fluctuations in the resonance frequency due to cutting errors when cutting off the dielectric sheet can be decreased. The strip line resonator electrodes 11a, 11b and input and output terminals 14a, 14b are coupled capacitively through the capacity electrodes 12a, 12b at the open ends of the strip line resonator electrodes. In the capacitive coupling method, as compared with the magnetic field coupling method generally employed in comb-line filters, since the coupling line is not necessary, the filter can be reduced in size. Application of the capacitive coupling method in this filter structure is accomplished for the first time by the establishment of the design method mentioned below. Another feature is that only a small capacity is enough for the coupling capacity because of coupling at open ends. A shield electrode 13a is formed on the dielectric sheet 10b, and a shield electrode 13b is formed on the dielectric sheet 10d. Each shield electrode is grounded by the grounding terminals 15a, 15b, 15c, 15d formed on the side electrodes. In the structure of the invention, the entire filter is covered with the shield electrodes, and hence the filter characteristic is hardly affected by external effects. By laminating the dielectric sheet 10e for electrode protection and laminating all other dielectric sheets, an entirely laminated structure is formed. Using a dielectric material of, for example, Bi-Ca-Nb-O ceramics with dielectric constant of 58 disclosed in H. Kagata et al.: "Low-fire Microwave Dielectric Ceramics and Multilayer Devices with Silver Internal Electrode," Ceramic Transactions, Vol. 32, The American Ceramic Society Inc., pp. 81-90, or other ceramic materials that can be baked at 950 degrees C or less, a green sheet is formed, and an electrode pattern is printed with metal paste of high electric conductivity such as silver, copper and gold, thereby laminating and baking integrally. In this way, when the laminate structure is formed by using the strip line resonators, the thickness can be reduced significantly. Operation of the thus constituted dielectric filter is described by reference to Fig. 1 and Fig. 2. Fig. 2 shows an equivalent circuit diagram of the dielectric filter in the first embodiment. The filter transmission characteristic in Fig. 2 can be calculated by using the even/odd mode impedance of the parallel coupling transmission line. In Fig. 2, reference numerals 21, 22 are input and output terminals, 17a, 17b are first transmission lines of the strip line resonator, 18a, 18b are second transmission lines of the strip line resonator, and capacitors 23, 24 are input and output coupling capacitors located between the strip line resonator electrodes 11a, 11b, and capacity electrodes 12a, 12b. In the case of a two-stage filter or a two-pole filter, the filter designing method in the first embodiment of the invention is described below. The even/odd mode impedances of the first transmission lines are supposd to be Z e1 , Z o1 , and the even/mode impedances of the second transmission lines to be Z e2 , Z o2 . The four-port impedance matrix of each transmission line is given in formula (1) by referring to, for example, the literature (T. Ishizaki et al., "A Very Small Dielectric Planar Filter for Portable Telephones": 1993 IEEE MTT-S, Digest H-1). Therefore, the two-port admittance matrix of two-terminal pair circuit 25 is newly calculated as in formula (2) for the structure of the invention, by connecting them in cascade, grounding one end, and using the other end as an input and output terminal. However, the line length of the first transmission lines and second transmission lines is set at the same line length L. By equalizing the line length, not only can the resonator length be set to the shortest, but also a very complicated calculation formula can be summarized into a simple form, thereby making it possible to design analytically. K e , K o , α, β, and t' are defined in formula (3). Where L is the line length of first transmission line or second transmission line, c is the velocity of light, and k is the propagation velocity ratio. To design a filter, first, from the design specification, the center frequency f o , attenuation pole frequency f p , bandwidth bw, and in-band ripple L r are determined. From these values, the value of g necessary for filter design is determined, and therefore the interstage admittance Y 3 and the shunt admittance of the modified admittance inverter Y 01 e , and input and output coupling capacities (C 01 ) 23, 24 are determined. Calculation of g, Y 3 , Y 01 e ,C 01 is shown in the literature (G.L. Matthaei et al., "Microwave Filters, Impedance-Matching Networks, and Coupling Structures": McGraw-Hill, 1964). Herein, t' in formula (3), replacing f with f o or f p , is defined as t' o , t' p . Therefore, the formulas necessary for realizing the filter characteristic to be designed are formula (4) for giving the attenuation pole frequency f p , Formula (4) t' 2 p = K o α - K e β α - β formula (5) for giving the filter center frequency f o , Formula (5) Y e 01 = j K o α + K e β - (α + β) t' 2 o 2αβ t' o and formula (6) for giving the interstage admittance Y 3 . Formula (6) Y 3 = - j K o α - K e β - (α - β) t' 2 o 2αβ t' o The solution that satisfies these three formulas simultaneously is the design value of the dielectric filter in Example 1 of the invention. Next, considering the structural parameters of the strip line, Z e1 and Z e2 , that is, Z e1 and K e (=Z e2 /Z e1 ) are properly determined. From formula (2) and formula (3), β can be eliminated, and t' o and t' p are determined. Hence, the line length L of each transmission line is determined. If the loading capacity is present at the open end of the strip line, formula (5) can be changed to formula (7) in the filter design formula. Formula (7) Y e 01 + Y L = j K o α + K e β - (α + β) t' 2 o 2αβ t' o where Y L is the admittance due to loading capacity. A design example of the filter of the embodiment is shown. Table 1 shows circuit parameter design values, with the center frequency f o of 1000 MHz, bandwidth bw of 50 MHz, in-band ripple L r of 0.2 dB, and attenuation pole frequency f p of 800 MHz in a first trial filter, and 1200 MHz in a second trial filter. Circuit parameter design values First filter Second filter Z e1 20Ω 20Ω Z 01 18.46Ω 14.88Ω Z e2 10Ω 10Ω Z 02 7.02Ω 7.41Ω L 3.00mm 3.20mm C 01 1.34pF 1.34pF Herein, the dielectric constant of the dielectric sheet is 58, and hence k is 0.131, Z e1 is 20Ω, and K e is 0.5. The loading capacity due to the discontinuous part at the open end is estimated at 3 pF. For an arbitrary value of the even mode impedance step ratio K e , the relation between K e and normalized resonator line length S is as shown in Fig. 3. The normalized resonator line length S is the value of the resonator line length of the filter divided by a quarter wavelength of the propagation wavelength. In the filter of the embodiment, in this way, by designing the resonator in the SIR structure, the line length can be set shorter than the quarter wavelength if loading capacity is not available, so that the filter can be reduced in size. That is, the resonator line length is shorter when the even mode impedance step ratio K e is smaller. Moreover, the relation of K e with the even/odd mode impedance ratio P 1 (=Z e1 /Z o1 ) of the first transmission line and the even/odd mode impedance ratio P 2 (=Z e2 /Z o2 ) of the second transmission line is shown in Fig. 4. The larger the value of K e , the larger the even/odd mode impedance ratio P 2 of the second transmission line, and hence the gap between the strip line resonators must be decreased, which is more difficult. On the other hand, if K e is small, the even mode impedance Z e1 of the first transmission line is considerably high, and the line width of the strip line may be narrower, which is also difficult to accomplish. To realize a favorable filter characteristic in the constitution of the embodiment, as determined from Fig. 4, the even/odd mode impedance ratio P 1 of the first transmission line and the even/odd mode impedance ratio P 2 of the second transmission line must be 1.05 or more and 1.1 or more respectively. Fig. 5 is a design chart for explaining the relation between the even mode impedance Z e and even/odd mode impedance ratio P as the parameter of strip line structure. In Fig. 5, at the dielectric constant of 58, the thickness of the dielectric sheet between strip line and upper and lower shield electrodes of 0.8 mm respectively, is calculated by varying the line width w of the strip line from 0.2 mm to 2.0 mm, and the gap between parallel strip lines from 0.1 mm to 2.0 mm. Fig. 5 enables checking whether the even/odd mode impedance ratio P of the transmission lines in Fig. 4 can be obtained. As a result, the value of the structural parameter for realizing the circuit parameter in Table 1 is determined as shown in Table 2 by referring to Fig. 5. Structural parameter design values First filter Second filter W 1 0.35mm 0.44mm g 1 1.22mm 0.54mm W 2 1.55mm 1.51mm g 2 0.20mm 0.27mm In the design in Table 2, the even/odd mode impedance ratio P of the transmission line is adjusted by varying the line distance, that is, the gap g. The coupling degree adjustment by the line distance is possible only by varying the electrode pattern, and it is easier to realize by far as compared with the method of, for example, varying the thickness of the dielectric sheet, and it is advantageous that the unloaded Q value of the resonator does not deteriorate. Fig. 6 is a graph showing the simulation results of the design value of transmission characteristic of the dielectric filter in the first embodiment. Fig. 7 shows the characteristic of the trial production of the filter of the embodiment, in which the solid line shows the measured value, and the broken line shows the calculated value about the actual dimensions of the trial product. In both diagrams, (a) shows the characteristic of the first trial filter with a low-zero, and (b) shows the characteristic of the second trial filter with a high-zero. These diagrams indicate that an attenuation pole is generated at the design frequency. The invention attains a novel effect of realizing superior selectivity by mutual electromagnetic coupling of the first transmission lines and second transmission lines of the resonator of the SIR structure, thereby not only shortening the resonator length, but also forming an attenuation pole at the design frequency. Thus, according to the embodiment, at least two or more TEM mode resonators are comprised in the SIR (stepped impedance resonator) structure with the overall line length shorter than a quarter wavelength constituted by cascade connection of other ends of the first transmission lines having one end grounded and the second transmission lines having one end open with the characteristic impedance lower than that of the first transmission lines. The first transmission lines are coupled electromagnetically, and the second transmission lines are coupled electromagnetically, and both electromagnetic field coupling amounts are set independently, and therefore a passing band and an attenuation pole are generated in the transmission characteristic, thereby realizing a small dielectric filter having a high selectivity. In this embodiment, a strip line resonator is shown, but a resonator of any structure may be used as far as it is a TEM mode resonator, and it is the same in the following examples. A laminated dielectric filter in a modified Example 1 of the invention is described below with reference to a drawing. Fig. 8 is a perspective exploded view of the laminated dielectric filter showing a modified first example of the invention. In Fig. 8, those same as the constitution in Fig. 1 are identified with the same reference numerals. The operating principle of this embodiment is the same as in the first embodiment. This embodiment differs from the first embodiment shown in Fig. 1 in that capacity electrodes 29a, 29b are formed on the dielectric sheet 10a, the same as the strip line resonator electrode layer. Accordingly, the dielectric sheet 10c in the first embodiment is not necessary, and the number of times of printing of the electrodes can be reduced by one, and it is free from the control of the thickness of the dielectric sheet 10c which is a cause of fluctuation in filter characteristic. Moreover, by forming a capacitor comprised of a capacity electrode as an interdigital type capacitor, a large capacity can be obtained easily, so that a wide range characteristic can be also realized. Example 2 A laminated dielectric filter in a second embodiment of the invention is described below with reference to a drawing. Fig. 9 is a perspective exploded view of the laminated dielectric filter. In Fig. 9, those structure that are the same as in Fig. 1 are identified with same reference numerals. What differs from Fig. 1 is that a loading capacity electrode 19 is provided so as to confront the open end portion of the strip line resonator electrodes 11a and 11b. In this embodiment, the resonance frequency can be further lowered by inserting the loading capacitor parallelly to the strip line resonator. As the filter design formula in this embodiment, formula (4) and formula (6) are the same as in Example 1, and only formula (5) is changed to the above described formula (7). Fig. 10 is a graph for explaining the relation between the loading capacity and resonator line length in the second embodiment. By adding the loading capacity, it is known that the resonator line length is further shortened. Thus, by providing the loading capacity electrode 19 confronting the open end portion of the strip line resonator electrodes 11a and 11b, the length of the resonator line can be further shortened, and the filter size can be reduced. Example 3 A laminated dielectric filter in a third embodiment of the invention is described below referring to the drawings. Fig. 11 is a perspective exploded view of the laminated dielectric showing the third embodiment of the invention. Fig. 12 is an equivalent circuit diagram of the laminated dielectric filter of the third embodiment. In Fig. 11, those structures same as in the structures in Fig. 1 are identified with same reference numerals. This embodiment differs from the first embodiment in Fig. 1 in that the coupling capacity electrode 20 and loading capacity electrode 19 are provided confronting the open end portion of the strip line resonator electrodes 11a, 11b. Prior to describing the operation of the dielectric filter of the embodiment, the difficulty in forming the attenuation pole near the passing band in the first embodiment is explained. Fig. 13 (a) and (b) are graphs showing the even/odd mode impedance ratio necessary for the attenuation pole frequency of the dielectric filter in the first embodiment. Fig. 13 (a) shows the filter with a low-zero, and Fig. 13 (b) shows the filter with a high-zero. As the attenuation pole frequency approaches the center frequency, the required even/odd mode impedance ratios P 1 , P 2 become larger. As the guideline for manufacture of actual filter, supposing the minimum value of the manufacturable line width w and gap g to be 0.2 mm, and their maximum value due to the request of the size of the filter to be 2 mm, the even mode impedance Z e that can be realized is in the range of 7 Ω to 35 Ω as shown in Fig. 5. That is, the minimum even mode impedance step ratio K e is 0.2. Moreover, if K e is large, the resonator length cannot be shortened, and hence there is a proper range for K e , and in relation to the structural parameter of the strip line, it is preferably 0.2 to 0.8, and more preferably 0.4 to 0.6. Hence, the even/odd mode impedance ratio P that can be realized is about 1.4 or less when the even mode impedance is 7 Ω, 1.9 or less at 20 Ω, and 2.2 or less at 35 Ω. Limitations on these values are restrictions on how closely the attenuation pole can be brought to the vicinity of the center frequency. In Fig. 13(a) and (b), based on the condition of P 2 being 1.4 or less, in the dielectric filter of the first embodiment, it is determined that the highest frequency of the lower attenuation pole frequency is 814 MHz, and the lowest frequency of the upper attenuation pole frequency is 1154 MHz. To alleviate these limitations, the coupling capacity and loading capacity are introduced, and the result is the dielectric filter of the third embodiment of the invention shown in Fig. 11. The operations of the laminated dielectric filter of the third embodiment is described referring to Fig. 11 and Fig 12. The transmission characteristic of the filter in the third embodiment shown in Fig. 12 can be calculated the same as in the filter in the first embodiment in Fig. 2 by using the even/odd mode impedance of the parallel coupling transmission line. In Fig. 12, those structures that are the same as in Fig. 2 are identified with the same reference numerals. What differs from Fig. 2 is that a coupling capacity (C C ) 28 formed between coupling capacity electrode 20 and strip line resonator electrodes 11a, 11b, and loading capacities (C L ) 26, 27 formed between the loading capacity electrode 19 and strip line resonator electrodes 11a, 11b are added. Concerning the two-pole filter of the third embodiment, a designing method is described below. The two-port admittance of the two-terminal pair circuit 25 of parallel coupling SIR resonator is given in formula (2) as mentioned above. Therefore, in the structure of the embodiment, as the formula necessary for realizing the design filter characteristic, the formulas (4), (5), (6) given in the first embodiment should be rewritten as follows. That is, the formula (8) for giving the attenuation pole frequency f p . Formula (8) t' p = - αβY C + (α - β) (K o α - K e β) + α 2 β 2 Y 2 C α - β the formula (9) for giving the filter center frequency f o , Formula (9) Y e 01 + Y L + Y C = j K o α + K e β - (α + β) t' 2 o 2αβ t' o and the formula (10) for giving the interstage admittance Y 3 . Formula (10) Y 3 - Y C = - j K o α - K e β - (α - β) t' 2 o 2αβ t' o The solution that satisfies these three formulas simultaneously is the design value of the dielectric filter of the fourth embodiment of the invention. The relation of the coupling capacity C C of the dielectric filter with a low-zero in the fourth embodiment with the corresponding even/odd mode impedance ratio (P 1 , P 2 ) and normalized resonator line length S is shown in Fig. 14. The relation of the loading capacity C L with the even/odd mode impedance ratio (P 1 , P 2 ) and normalized resonator length S is shown in Fig. 15. These diagrams are calculated at the center frequency f o of 1000 MHz, attenuation pole frequency f p of 800 MHz, and even mode impedance step ratio K e of 0.2. In Fig. 15, the loading capacities (C L ) 26, 27 are fixed at 0 pF, and in Fig. 16 the coupling capacity (C C ) 28 is fixed at 0 pF. When the coupling capacity C C increases, P 1 increases, P 2 decreases, and S is unchanged. On the other hand, when the loading capacity C L increases, P 1 decreases, P 2 increases, and S decreases. Therefore, by the combination of the coupling capacity (C C ) 28 and loading capacities (C L ) 26, 27, the even/odd mode impedance ratio (P 1 , P 2 ) can be adjusted to a practical value. Hence, an attenuation pole may be made up in the vicinity of the passing band. Fig. 13 (a), shows that when the even/odd mode impedance ratio P 1 of the first transmission lines is smaller than the even/odd mode impedance ratio P 2 of the second transmission lines, a low-zero is formed in the dielectric filter in the first embodiment. When the even/odd mode impedance ratio P 1 of the first transmission lines is larger than the even/odd mode impedance ratio P 2 of the second transmission lines, Fig.13 (a) shows that a high-zero is formed in the dielectric filter in the first embodiment. On the other hand, Figs. 14, 15 of the third embodiment show the possibitity that their relation may be exchanged depending on the magnitude of the coupling capacity and loading capacity. Therefore, by thus properly setting the relation of P 1 and P 2 , the attenuation pole can be freely formed at a specified frequency in the structure of the invention. Fig. 16 (a) is a graph showing the minimum required coupling capacity and loading capacity values for the attenuation pole frequency of the dielectric filter possessing the low-zero in the third embodiment. Fig. 16 (b) is a graph showing the minimum required coupling capacity and loading capacity values for the attenuation pole frequency of the dielectric filter with a high-zero in the third embodiment. As known from the curves of the graphs, although not created by the dielectric filter of the structure in the first embodiment, the attenuation pole in a frequency range of within 15% on both sides of the polarity of the center frequency, specifically the attenuation pole in a frequency range of 814 MHz to 1154 MHz can be manufactured in the dielectric filter of the structure in the third embodiment. It is also shown that the loading capacity is essential in the close vicinity to the passing band. By forming an attenuation pole in the frequency range of within 15% on both sides of the polarity of the center frequency, a band pass filter having a high selectivity can be realized. Fig. 17 (a) and (b) are graphs showing the transmission characteristic simulation result for improving the attenuation amount near the passing band of the dielectric filter in the first embodiment and fourth embodiment. Fig. 17 (a) relates to a filter with low-zero, and Fig. 17 (b) shows a filter with a high-zero. In both cases, the solid line shows the characteristic when the attenuation pole is brought closest to the passing band in the filter of the first embodiment, and the broken line shows the characteristic obtained in the filter of the third embodiment. In the filter of the fourth embodiment, a superior selectivity characteristic to that of the filter of the first embodiment is obtained. Thus, this embodiment comprises at least two or more TEM mode resonators in the SIR (stepped impedance resonator) structure with an overall line length shorter than a quarter wavelength constituted by cascade connection of other ends of the first transmission lines having one end grounded and the second transmission lines having one end open with the characteristic impedance lower than that of the first transmission lines. The first transmission lines are coupled electromagnetically, and the second transmission lines are coupled electromagnetically. Both electromagnetic coupling amounts are set independently, while at least two TEM mode resonators are capacitively coupled through separate coupling means, so that an attenuation pole can be generated near the passing band of transmission characteristic, which is an excellent characteristic. Also, in the third embodiment, by inserting the loading capacity parallelly to the strip line resonator, the resonator line length can be further shortened, and therefore the filter can be reduced in size. Therefore, a small dielectric filter with high selectivity can be realized. Such characteristic is very preferable for a high frequency filter for use in, for example, a portable telephone. Example 4 Referring now to the drawings, a laminated dielectric filter in a fourth embodiment of the invention is described below. Fig. 18 (a) is a perspective exploded view of the laminated dielectric filter showing the fourth embodiment of the invention, and Fig. 18 (b) is a sectional view of section A-A' of the laminated dielectric filter showing the fourth embodiment of the invention. Fig. 19 is an equivalent circuit diagram for description of the operation in the laminated dielectric filter of the fourth embodiment shown in Fig. 18. The filter circuit constitution of the embodiment has many points common with the third embodiment in appearance. However, each resonator is not necessarily required to be in SIR structure composed of the first transmission line and the second transmission line lower in characteristic impedance than the first transmission line. Therefore, in the constitution of the embodiment, an independent electromagnetic field coupling amount of the first transmission lines or second transmission lines is not taken into consideration at all. In Fig. 18, reference numerals 200a, 200b are thick dielectric sheets. Strip line resonator electrodes 201a, 201b are formed on the dielectric sheet 200a, and a second electrode 202a, a third electrode 202b, and fourth electrodes 202c, 202d of a parallel flat plate capacitor are formed on the dielectric sheet 200c. A shield electrode 203a is formed on the dielectric sheet. 200b, and a shield electrode 203b is formed on the dielectric sheet 200d. A dielectric sheet 200e for the protection of the electrode is laminated together with all other dielectric sheets, and an entirely laminated structure is formed. As the dielectric material, for example, ceramics of Bi-Ca-Nb-O system with the dielectric constant of 58, or other ceramic material that can be baked at 950 °C or less can be used. A green sheet is formed, and an electrode pattern is printed by using metal paste of high electric conductivity such as silver, copper and gold, and the materials are laminated and baked into one body. By baking, the dielectric sheets and electrode layers shrink and contract by about 10 to 20% in the horizontal direction and vertical direction. If the multiple factor of the shrinkage of the electrode layer is larger than that of the dielectric sheet, the terminal of the electrode is indented inward at the end of the laminate, and it cannot be connected with the terminal electrode formed on the side. To avoid this, using an electrode material in which the multiple factor of the shrinkage in baking is slightly smaller than that of the dielectric sheet, strip line resonator electrodes and shield electrodes are formed on respective dielectric sheets, and the dielectric sheets are laminated and baked into one body. In this way, the electrode terminal is projected to the end face of the laminate by several to scores of micrometers, thus attaining a successful connection with the terminal electrode formed on the side. The thick dielectric sheets 200a, 200b can be formed into a specified thickness by laminating a plurality of thin green sheets. Thus, all dielectric sheets can be formed in a normalized thickness, so that it is easy to manufacture. The fourth electrodes 202c, 202d are connected to side electrodes 204a, 204b of the input and output terminals. The upper and lower shield electrodes 203a, 203b are connected to the side electrodes 205a, 205b of the grounding terminals. The side electrodes at grounding terminals are grounded by providing at two side surfaces of the strip line resonator, that is, the side surface of the open end and the side surface at the short-circuit end, thereby suppressing the resonance of the shield electrodes and preventing deterioration in filter characteristic. Moreover, by forming a side electrode 205a as the grounding terminal between the input terminal and output terminal, it is effective to isolate between the input and output terminals. By forming asymmetrically by varying the number or shape of the side electrodes provided at the side surfaces, the mounting direction of the laminated dielectric filter can be easily recognized. The shape of the shield electrodes 203a, 203b is formed by leaving a marginal blank space so that the outer periphery of the shield electrode may settle within the outer periphery of the dielectric sheet, except for the connecting position of the side electrode as a grounding terminal and its surroundings, forming the shield electrode one size smaller than the dielectric sheet. The adhesion strength of the green sheets of laminated ceramics is weak in the holding area of the metal paste for forming the electrode pattern, and particularly in the outer periphery of the dielectric sheet, a blank space of the shield electrode is provided so that the ceramics may adhere directly with each other. Besides, by forming two layers of shield electrodes in the same shape, one kind of screen is sufficient for printing a shield electrode pattern. Moreover, by forming both upper and lower layers of the shield electrodes with the inner layer electrode, the forming method is the same as in the strip line resonator electrode layer and capacity electrode layer, so that manufacturing is easy. On the uppermost layer, by laminating the dielectric sheet 200e for protecting the electrode, it is possible to protect the upper shield electrode layer 203a formed of an inner layer electrode that is not sufficient in mechanical strength. Of course, since the lower shield electrode layer 203b is also printed on the dielectric sheet 200d, it is protected from the external environment. The strip line resonator is reduced in size by narrowing the line width of the short-circuit side of the strip line in the midst of the strip line, in steps from the broad parts 211a, 211b to the narrow parts 212a, 211b. The short-circuit side of the electrodes 212a, 212b at the narrow side of the strip line resonator is connected to the side electrode 205b of the grounding terminal through the broad common grounding electrode 213, and is grounded. The length change of the broad common grounding electrode 213 has a smaller effect on the resonance frequency than the length change of the strip line resonator electrodes 201a, 201b, and therefore it is possible to suppress the fluctuations in resonance frequency due to variations when cutting off the dielectric sheet. In this embodiment, the line width of the strip line resonator is changed in steps on the way toward the strip line. But different from the first to fifth embodiments, the strip line resonator having a constant line width may be also used. Other modifications such as slope change of line width may be also be applicable. The operation of thus formed laminated dielectric filter in the embodiment of the invention is described below referring to Figs. 18 (a), 18 (b) and 19. First, the strip line resonator electrodes 201a, 201b, and the second, third and fourth electrodes 202a, 202b, 202c, 202d respectively have parallel flat plate capacitors 221, 222, 223, 223, 225, 226 between them. The parallel flat plate capacitor 221 between the second electrode 202a and strip line resonator electrode 201a, and the parallel flat plate capacitor 222 between the second electrode 202a and strip line resonator electrode 201b function as interstage coupling capacitors. Therefore, the interstage coupling between resonators is achieved by the combination of electromagnetic field coupling between strip line resonators and electric field coupling through the parallel flat plate capacitors 221 and 222 connected in series.. When the distance between the strip line resonator electrodes is shortened for reduction of size, usually, the interstage coupling by electromagnetic field coupling becomes too large, and it is hard to realize a favorable narrow band characteristic. However, in the constitution of the invention, the interstage coupling can be reduced by cancellation of couplings by the combination of electromagnetic field coupling and electric field coupling, and a narrow band characteristic can be realized. At the same time, by the resonance phenomenon by combination of electromagnetic field coupling and electric field coupling, an attenuation pole can be composed in the transmission characteristic, so that excellent selectivity characteristic may be obtained. What is of note here is that the generation method of the attenuation pole in the transmission characteristic is radically different from the generation method of attenuation pole in the dielectric filters in the first to fourth embodiments. That is, in the dielectric filters of the first to fifth embodiments, the first transmission lines and the second transmissions lines of the resonator in SIR structure are mutually coupled electromagnetically, whereas, in the constitution of this embodiment, the attenuation pole is generated by the parallel resonance by the combination of electromagnetic field coupling between resonators and electric field coupling due to interstage coupling capacitor. The principle of generation of attenuation pole in the embodiment is described specifically in JP-A-5-95202 and T. Ishizaki et al., "A Very Small Dielectric Planar Filter for Portable Telephones," 1993, IEEE MTT-S Digest, H-1, pp. 177-180, 1993. The related technology is also disclosed in United States Patent No. 4,742,562 and R. Pregla, "Microwave Filters of Coupled Lines and Lumped Capacitances," IEEE Trans. on Microwave Theory and Tech.,Vol. MTT-18, No. 5, pp. 278-280, May 1970. The capacity electrode of the interstage coupling capacitor is composed of a second electrode 202a which is a floating electrode not electrically connected to any terminal electrode provided in the capacity electrode layer. The feature of this embodiment is that the electrode surface 201a and 201b of the strip line resonator are used dualistically as the first electrode for the comprising the parallel flat plate capacitor, and the parallel flat plate capacitors 221, 222 are connected in series, thereby realizing the interstage coupling capacitor in a flat laminatable structure. The parallel flat plate capacitor 223 located between the third electrode 202b and the strip line resonator electrode 201a, and the parallel flat plate capacitor 224 located between the third electrode 202b and strip line resonator electrode 201b function as parallel loading capacitors for lowering the resonance frequency of the strip line resonator. Therefore, the length of the strip line resonators 201a, 201b can be set shorter than a quarter wavelength, so that the filter size can be reduced. In Fig. 18, the third electrode 202b is integrated to confront the both two strip line resonator electrodes 201a and 201b, but the third electrode 202b may be separated into two divisions, and the third electrode may be independently provided and grounded in the strip line resonator electrodes 201a and 201b. The parallel flat plate capacitor 225 disposed between the fourth electrode 202c and the strip line resonator electrode 201a, and the parallel flat plate capacitor 226 disposed between the fourth electrode 202d and strip line resonator electrode 201b function as input and output coupling capacitors. In the constitution of the embodiment, since the shield electrode layer and capacity electrode layer are composed of different layers, a large coupling capacity may be formed between the strip line resonator electrode and capacity electrode, while keeping thick the thickness of the dielectric sheet between the strip line resonator electrode and shield electrode, so that a large capacity may be used for input and output coupling or interstage coupling. Supposing, for example, the capacity electrode is positioned in the same layer as the shield electrode layer, the dielectric sheet between the shield electrode layer and capacity electrode layer must be thin, the unloaded Q value deteriorates, and it is very difficult to realize a required coupling degree in the filter of the invention. However, in the constitution of the invention, the capacity electrode layer formed separately from the shield electrode layer is confronting the strip line resonator electrode layer across the thin dielectric sheet, thereby efficiently solving the problem. In this constitution, moreover, all strip line resonator electrodes are printed on the dielectric sheet 200a, and all capacity electrodes on the dielectric sheets 200c, and hence electrode printing is required only in the dielectric sheet and the shield electrode layer, and the number of printing steps is small and fluctuations in filter characteristic may be suppressed. That is, by placing the strip line resonator electrode layer in one electrode layer, the relative positional precision between the strip line resonator electrodes can be improved, so that fluctuations may be reduced. Additionally, by forming the capacity electrode layer in one layer in electrode layer, control of the thickness of dielectric sheet which has a large effect on the characteristic fluctuations of the filter is effected by only controlling one layer of dielectric sheet 200c between the strip line resonator electrode layer and the capacity electrode layer, so that manufacturing control is very easy, which is another great advantage. Fig. 20 is a configuration perspective view of the capacity electrodes and strip line resonator electrodes of the laminated dielectric filter in the fourth embodiment of the invention. In the manufacturing processing of the laminated dielectric filter, it may be considered that the filter characteristic may fluctuate due to deviation in the position of the strip line resonator electrode layer and capacity electrode layer. To eliminate such effect, as shown in Fig. 20, in the overlapping region of each capacity electrode with the outer edge of the strip line resonator electrode, a dent is formed in the capacity electrode to narrow the width of the electrode. A dent 231 is formed in the second electrode 202a, dents 232, 233, 234 are formed in the third electrode 202b, and dents 235, 236 are formed in the fourth embodiments 202c, 202d. By forming such narrow dent regions, the change in the area of the overlapping regions when position deviation occurs between the strip line resonator electrode layer and capacity electrode layer may be set considerably smaller as compared with the case without dents. Meanwhile, as shown in the electrode configuration in Fig. 20, the electrode 202a of the interstage coupling capacitor is positioned between the open end and short-circuit end, not between the open ends of the strip line resonator electrodes 201a, 201b, because of the convenience of the electrode pattern layout, and it is different from the equivalent circuit in Fig. 19. When the position of the interstage coupling capacitor is moved from the open end to the short-circuit end, it has the same effect as decreasing the capacitance of the interstage coupling capacitor, equivalently. That is, the frequency of the attenuation pole moves to the higher side, and is deviated from the design value. However, for the convenience of description of the operation of the filter, the equivalent circuit in Fig. 19 is shown. Example 5 A laminated dielectric filter of a fifth embodiment of the invention is described by reference to a drawing. Fig. 21 is a perspective exploded view of a laminated dielectric filter in the seventh embodiment of the invention. In Fig. 21, the same elements as in Fig. 18 are identified with same reference numerals. What differs from the fourth embodiment is that the fourth electrodes 202e, 202f taken out from the lateral direction of the strip line resonator electrode are used instead of the fourth electrodes 202c, 202d in the sixth embodiment. In this relation, the side electrodes as input and output terminals are changed from 204a, 204b to 204c, 204d, and the side electrode as a grounding terminal is changed from 205a to 205c. By taking out the fourth electrodes as the input and output electrodes from the lateral direction, the distance between the input and output electrodes can be extended, and hence the spatial coupling between input and output can be decreased, so that the isolation can be wider. In the fifth embodiment, the coupling position of the fourth electrodes is between the open end and short-circuit end of the strip line resonator electrodes. The equivalent circuit diagram of the laminated dielectric filter of the seventh embodiment is shown in Fig. 22. The input and output coupling capacitors 225, 226 are tapped down, and connected to the strip line resonator. Therefore, the broad parts 211a and 211b of the strip line resonator electrodes can be separately considered for the electrodes 213a and 214a, and 213b and 214b. Herein, the series circuit 251 composed of electrode 213a and loading capacitor 223, and the series circuit 252 composed of electrode 213b and loading capacitor 224 both function as series resonance circuits. At the resonating frequency of the series circuits 251, 252, the impedance is zero, and hence an attenuation pole is formed in the filter transmission characteristic. That is, in the fifth embodiment, aside from the attenuation pole produced by the combination of electromagnetic field coupling and electric field coupling of the resonator in the fourth embodiment, the attenuation pole is also produced by the series resonance of the series circuits 251, 252, so that an excellent selectivity characteristic may be obtained. Example 6 A laminated dielectric filter in a sixth embodiment of the invention is described below with reference to the accompanying drawings. Fig. 23 is a perspective exploded view of the laminated dielectric filter showing the sixth embodiment of the invention. In Fig. 23, the same constituent elements as in Fig. 18 and Fig. 21 are identified with the same reference numerals. Fig. 24 is an equivalent circuit diagram for explaining the operation of the laminated dielectric filter in the eighth embodiment shown in Fig. 23. The sixth embodiment differs from the fifth embodiment in that the filter is composed of three stages. Strip line resonator electrodes 261a, 261b, 261c are respectively composed of broad parts 2141, 214b, 214c, and narrow parts 215a, 215b, 215c, and the short-circuit side of the narrow parts is connected and grounded to the side electrode 205b as the grounding terminal through a broad common grounding electrode 216. The second electrode 262a is formed on the dielectric sheet 200c, partly confronting all of the strip line resonator electrodes 261a, 261b, 261c, thereby realizing the interstage electric field coupling. In the regions contacting the strip line resonator electrodes on the dielectric sheet 200c, the third electrode 262b is formed and grounded partly in the remaining region of the second electrode. The parallel flat plate capacitor composed between the third electrode 262b and the strip line resonator electrode functions as the parallel loading capacitor for lowering the resonance frequency of the strip line resonator. Therefore, the length of the strip line resonators 261a, 261b, 261c can be cut shorter than the quarter wavelength, so that the filter size can be reduced. The shield electrodes 263a, 263b are formed on the dielectric sheets 200b, 200d so as to cover entirely over. By laminating the dielectric sheets 200e for protecting the electrode on the uppermost layer, it is possible to protect the upper shield electrode layer 263b formed of an inner layer electrode that not sufficient in the mechanical strength. In this embodiment, since the coupling position of the fourth electrode is located between the open end and short-circuit end of the strip line resonator electrodes, the equivalent circuit diagram of the laminated dielectric filter of the embodiment is as shown in Fig. 24. The input and output capacitors 225, 226 are tapped down, and connected to the strip line resonator. Therefore, the broad parts 214a, 214b of the strip line resonator electrodes can be considered separately for the electrodes 217a and 218a, and 217b and 218b. At the resonating frequency of the series circuit 277 composed of the electrode 217a and loading capacitor 274, and the series circuit 278 of the electrode 217b and loading capacitor 275, an attenuation pole is formed in the filter transmission characteristic. It is same as in the fifth embodiment. The mutually adjacent strip line resonators are coupled electromagnetically, and are also coupled electrically through the interstage coupling capacitors 271, 272, 273, and by coupling the strip line resonators by the combination of electromagnetic field coupling and electric field coupling, two attenuation poles can be composed in the transmission characteristic by the resonance phenomenon by the combination of electromagnetic field coupling and electric field coupling, so that an excellent selectivity characteristic can be obtained. The basic constitution in the sixth embodiment can be the same as in the fifth embodiment, or it may be constituted the same as in the fourth embodiment by setting the take-out direction of the input and output terminals the same as the direction of the open end of the strip line resonator electrodes. Thus, in the sixth embodiment, by constituting the filter in three stages, excellent selectivity is obtained. The selectivity can be even further enhanced by composing in four or five stages. Example 7 Referring to the drawings, a laminated dielectric filter in a seventh embodiment of the invention is described below. Fig. 25 is a perspective exploded view of the laminated dielectric filter showing the seventh embodiment of the invention. In Fig. 25, the same constituent elements as in Figs. 18, 21, 23 are identified with the same reference numerals. Fig. 26 is an equivalent circuit diagram for explaining the operation of the laminated dielectric filter of the ninth embodiment shown in Fig. 25. The operation in the seventh embodiment is almost the same as in the sixth embodiment. The seventh embodiment differs from the eighth embodiment in the connecting method of the interstage coupling capacitor. In the sixth embodiment, the second electrode for forming the interstage coupling capacitor is composed of one electrode 262a confronting all strip line resonator electrodes, but in this embodiment, the second electrode is composed of the electrodes 281, 282 provided in every adjacent strip line resonator electrode. The adjacent strip line resonators are coupled in electromagnetic field, and are also coupled in electric field through the interstage coupling capacitor composed of capacitors 283 and 284, and 285 and 286 connected in series, and the strip line resonators are coupled by combination of electromagnetic field coupling and electric field coupling, and therefore two attenuation poles are composed in the transmission characteristic by the resonance phenomenon by combination of electromagnetic field coupling and electric field coupling. In this way, in the seventh embodiment, the same effect as in the sixth embodiment can be obtained, and the resonance characteristic can be designed by the combination of electromagnetic field coupling and electric field coupling in each adjacent strip line resonator, so that the design is easier than in the sixth embodiment.

Claims: A laminated dielectric filter comprising a strip line resonator electrode layer forming plural strip line resonators (11a, b), and a capacity electrode layer (12a, b), wherein the strip line resonator electrode layer and the capacity electrode layer are sandwiched by two shield electrode layers (13a, b), and the space between the two shield electronic layers are filled with a dielectric, and the thickness between the strip line resonator electrode layer (11a, b) and capacity electrode layer (12a, b) is set thinner than the thickness between the strip line resonator electrode layer (11a, b) and shield electrode layer (13b) and the thickness between the capacity electrode layer (12a, b) and shield electrode layers (13a), wherein the dielectric between the strip line resonator electrode layer and the shield electrode layer, and the dielectric between the capacity electrode layer and the shield electrode layer are respectively formed by laminating a plurality of thin dielectric sheets, characterized in that thick dielectric sheets (10a, b) are formed by laminating a plurality of the thin dielectric sheets, thus all dielectric sheets being formed of thin dielectric sheets of the same standardized thickness. The filter of claim 1, wherein the strip line resonaters (11a, b) have a front end short-circuit structure, and the short-circuit and is connected and grounded electrically to the grounding terminal formed at the side of the dielectric through a broad common grounding electrode (16) formed on the same electrode layer as the strip line resonator electrode layer (11a, b). The filter of claim 1 or 2, wherein an interstage coupling capacity electrode, or input and output coupling capacity electrode, or loading capacity electrode formed on the capacity electrode layer has a dent shape (231 ... 236) narrowed in the electrode width in the region overlapping with the outer edge of the strip line resonator electrodes (211a, b) of the strip line resonator electrode layer. The filter of any of claims 1 to 3, wherein the laminated dielectric filter has an input and output coupling capacity electrode (202e, f) on the capacity electrode layer, and the strip line resonator (211a, b) have a front end short-circuit structure, and moreover the input and output coupling capacity electrodes and strip line resonators are coupled capacitively at an intermediate position between the open end and short-circuit end of the strip line resonators. The laminated dielectric filter of claim 4, wherein input and output terminals (14a, b) electrically connected to the input and output coupling capacity electrode (12a, b) are formed of side electrodes provided in the lateral direction of the strip line resonator (11a, b). The filter of any of claims 1 to 5, wherein the multiple factor of the shrinkage in baking of the dielectric is set smaller than the shrinkage rate in baking of the electrode material for composing the strip line resonator electrode layer (11a, b) and capacity electrode layer (12a, b). A laminated dielectric filter comprising a strip line resonator electrode layer forming plural strip lines resonators (11a, b) and capacity electrode layer (29a, b), wherein the strip line resonator electrode layer and the capacity electrode layer are sandwiched by two shield electrode layers (13a, b) and the space between the two shield electronic layers are filled with a dielectric, wherein the dielectric between the strip line resonator electrode layer (11a, b) and the shield electrode layers (13a, b) and the dielectric between the capacity electrode layer (29a, b) and the shield electrode layers (13a, b) are respectively formed by laminating a plurality of thin electric sheets, characterised in that the capacity electrode (29a, b) and the strip line resonator electrode layer (11a, b) are formed on a same level on a same dielectric sheet (10a) and thick dielectric sheets (10a, b) are formed by laminating a plurality of the thin dielectric sheets, thus all dielectric sheets being formed of thin dielectric sheets of the same standardized thickness.

Inventors: AIZAWA KIMIO, FUJINO TAKASHI, ISHIZAKI TOSHIO, KUSHITANI HIROSHI, NAKAKUBO HIDEAKI, NAKAMURA TOSHIAKI, SASAKI ATSUSHI, SATOH YUKI, AIZAWA, KIMIO, FUJINO, TAKASHI, ISHIZAKI, TOSHIO, KUSHITANI, HIROSHI, NAKAKUBO, HIDEAKI, NAKAMURA, TOSHIAKI, SASAKI, ATSUSHI, SATOH, YUKI
Applicants: MATSUSHITA ELECTRIC IND CO LTD, MATSUSHITA ELECTRIC INDUSTRIAL CO., LTD.
Application year: 1994
Main classification: H01P1
IPC codes: H01P1